Instrumented indentation was performed to determine the statistics of discontinuous yield behavior of Co and Ni pre-strained to various levels. In both materials

increasing pre-strain decreases the frequency of indentations that exhibit discontinuous yield. Yield events in Co occurred at nominally the same stress independent of pre-strain

suggesting dislocation nucleation dominates during contact loading and that the existing dislocations are strongly pinned in Co. However

in Ni yield occurred at lower loads with increasing pre-strain

suggesting that dislocation activation

rather than true nucleation

dominates the yield mechanism.